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An Assessment of Alkaline Fuel Cell Technology

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An Assessment of Alkaline Fuel Cell Technology International Journal of Hydrogen Energy 27 (2002) 507–526 www.elsevier.com/locate/ijhydene An assessment of alkaline fuel cell technology G.F. McLean ∗, T. Niet, S. Prince-Richard, N. Djilali University of Victoria, POB 3055, STN CSC Victoria, BC Canada V8W 3P6 Abstract This paper provides a review of the state of the art of alkaline fuel cell (AFC) technology based on publications during the past twenty-ÿve years. Although popular in the 1970s and 1980s, the AFC has fallen out of favour with the technical community in the light of the rapid development of Proton Exchange Membrane Fuel Cells (PEMFCs). AFCs have been shown to provide high power densities and achieve long lifetimes in certain applications, and appear to compete favourably with ambient air PEM fuel cells. In this report we examine the overall technology of AFCs, and review published claims about power density and lifetime performance. Issues surrounding the sensitivity of the AFC to CO2 in the oxidant stream are reviewed and potential solutions discussed. A rough cost comparison between ambient air AFCs and PEMFCs is presented. Overall, it appears the Alkaline Fuel Cell continues to have potential to succeed in certain market niche applications, but tends to lack the R& D support required to reÿne the technology into successful market o<erings. ? 2002 Published by Elsevier Science Ltd on behalf of the International Association for Hydrogen Energy. 1. Introduction published information on AFCs to provide a uniÿed view of the technology. A re-examination of the economics of AFC The alkaline fuel cell (AFC) was the ÿrst fuel cell tech- technology is also presented. The issues generally assumed nology to be put into practical service and make the gen- to have caused the demise of interest in AFCs, namely low eration of electricity from hydrogen feasible. Starting with power density and electrolyte poisoning are addressed in applications in space the alkaline cell provided high-energy detail to provide as complete a picture as possible, based conversion e=ciency with no moving parts and high reli- primarily on published and publicly available information. ability. AFCs were used as the basis for the ÿrst experi- The PEM fuel cell has recently emerged as the technol- ments with vehicular applications of fuel cells, starting with ogy of choice for low temperature, moderate power applica- a farm tractor in the late 1950s equipped with an Allis tions and has largely displaced the AFC in this application. Chalmers AFC (Kordesch and Simader, 1996). This was Because of this, we have provided a comparison between followed by the now famous Austin A40 operated by Karl alkaline and PEM technology wherever possible. In partic- Kordesch in the early 1970s [1] and continuing today with ular, a detailed cost comparison between PEM and AFCs is the commercialization activities of the ZEVCO company included. [2,3]. However, despite its early success and leadership role The public domain literature has been reviewed includ- in fuel cell technology, AFCs have fallen out of favour ing the most recently published results on alkaline elec- with the research community and have been eclipsed by the trode materials and manufacture as well as older publications rapid development of the Proton Exchange Membrane fuel describing the state of the art around 1980. Earlier publica- cell (PEMFC) as the technology of choice for vehicular tions, which largely describe the now defunct space appli- applications. cations of AFC technology, have not been reviewed. The This paper provides a critical overview of the state of overall purpose of this review has been to establish a techni- the art of AFC technology and attempts to synthesize the cal opinion about the viability of AFCs and to identify key areas for research. The report is structured as follows. In Section 2 we pro- ∗ Corresponding author. Fax: +250-721-6323. vide a general orientation to AFC technology and review E-mail address: [email protected] (G.F. McLean). the nature of the published research and recent corporate 0360-3199/02/$ 20.00 ? 2002 Published by Elsevier Science Ltd on behalf of the International Association for Hydrogen Energy. PII: S 0360-3199(01)00181-1 508 G.F. McLean et al. / International Journal of Hydrogen Energy 27 (2002) 507–526 et al. [5] and De Geeter [4], the active layer consists of an organic mixture (carbon black, catalyst and PTFE) which is ground, and then rolled at room temperature to cross link the powder to form a self supporting sheet. The hydrophobic layer, which prevents the electrolyte from leaking into the reactant gas Low channels and ensures di<usion of the gases to the reaction site, is made by rolling a porous organic layer, again to cross-link the layer and form a self-supporting sheet. The two layers are then pressed onto a conducting metal mesh. The process is eventually completed by sintering. The total electrode thickness is of the order of 0.2–0:5mm. A major operating constraint is the requirement for low carbon dioxide concentrations in the feed oxidant stream. In the presence of CO2, carbonates form and precipitate; − 2− CO2 + 2OH → (CO3) +H2O: The carbonates can lead to potential blockage of the elec- trolyte pathways and=or electrode pores. This issue is dis- cussed in detail in Section 3.2.1. The inherently faster kinetics of the oxygen reduction re- action in an alkaline cell allows the use of non-noble metal Fig. 1. Alkaline fuel cell composition. electrocatalysts. It is useful to compare the eletrochemical performance of AFCs and PEMFCs in terms of the relation- ship between cell potential, E, and current density, i. When activities. In Section 3 we discuss the major technical issues mass transport limitations are negligible (low to intermedi- confronting AFCs, including the reported power densities, ate current density), E and i are approximately related by poisoning issues, lifetime, duty cycles and systems consid- Blomen and Mugerwa [6] erations. This section also provides a hint at some new AFC technologies that may be of interest. Section 4 provides a de- E = E0 − ÿ log i − Ri tailed cost analysis and includes a comparison to published PEM cost projections. In Section 5 we provide conclusions with and state our general technical position. E0 = Er + ÿ log i0; 2. Alkaline fuel cell background and development status where, Er is the reversible thermodynamic potential, ÿ and i0 are the Tafel slope and the exchange current density for 2.1. Principle of operation the oxygen reaction, and R is the di<erential resistance of the cell. AFCs use an aqueous solution of potassium hydroxide as Di<erentiating Eq. (1) provides further insight into the the electrolyte, with typical concentrations of about 30%. relative importance of losses associated with electrode ki- The overall chemical reactions are given by netics and electric resistance: @E ÿ − − = − − R: Anode reaction 2H2 + 4OH → 4H2O+4e @i i − − Cathode reaction O2 +2H2O+4e → 4OH At low current densities, the ÿrst term on the RHS is dom- Overall cell reaction 2H2 +O2 → 2H2O +electric energy + heat inant and corresponds to the typical steep fall of the cell potential with increasing current. At higher current densi- By-product water and heat have to be removed. This is ties, ÿ R and the second term becomes dominant, result- usually achieved by recirculating the electrolyte and using it ing in a quasi-linear drop of cell potential with current, until as the coolant liquid, while water is removed by evaporation. mass transport limitations become important. Optimal per- A schematic of the recirculating electrolyte AFC is shown formance is obtained for low Tafel slopes (ÿ) and cell resis- in Fig. 1 (after De Geeter [4]). tance (R), and high exchange current density (i0). The better The electrodes consist of a double layer structure: an ac- electrode kinetics of AFCs results in Tafel slopes lower by tive electrocatalyst layer, and a hydrophobic layer. Accord- about 30% than for PEMFC, when Pt is used as a catalyst ing to the dry manufacturing method described by Kivisaari in both [7]. G.F. McLean et al. / International Journal of Hydrogen Energy 27 (2002) 507–526 509 The main contribution to cell resistance is due to the and mobile fuel cell applications since the mid 1980s. Ma- ionic=protonic resistivity of the electrolyte. Again AFCs jor European projects conducted by Siemens, Hoechts and appear to have lower electrolyte resistivities (0.05 vs. DLR were all cancelled prior to 1996 [21]. North American 0:08 P=cm2 for PEMFC). It should be pointed out that development of AFCs for space applications is continued new generation ultra thin acidic polymer membranes [8] by United Technologies=International fuel Cells. However, achieve low resistance. Nonetheless AFCs have an intrin- this work appears to be limited to providing fuel cells to the sic advantage over PEMFC on both cathode kinetics and space shuttle program and appears to have no aspirations for ohmic polarization. A puzzling aspect of all published AFC entering other markets [22]. data is that polarization curves are invariably presented for The remaining developers of AFC technology are almost maximum current densities of about 400 mA=cm2, with exclusively related to Zetek Corporation. Zetek is the par- no indication that mass transport limitations have been ent organization of three companies involved in developing reached. A possible explanation is that, for cost reasons, products for transportation (Zevco plc), marine (ZeMar Ltd) the catalysts of choice are nickel alloys. Nickel is, how- and stationary power (ZeGen Ltd) applications. Recent de- ever, susceptible to oxidation, leading to high performance velopments from Zetek include the announcement of a new degradation over time. This problem would presumably be 5MW automated production line in Germany that will see exacerbated at higher current densities.
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